Electric arc welding has been employed in many applications. A task in electric arc welding is stabilizing the waveform of the DC welding current.
What follows is a description of a prior art of arc welding power source. The arc welding power source as referred to here is a power source for consumable electrode arc welding.
A consumable electrode arc welder needs a power source that is capable of supplying a large DC current reaching several hundreds of amperes. In order to obtain such a DC supply, a means to rectify three phases AC source is mainly used; and sometimes several rectifying circuits in parallel are used taking the characteristics of rectifiers into consideration. For example, Japan patent gazette Toku Kai Hei3-253262 (Japanese Unexamined Patent Publication 253262/1991) describes a means in which two outputs of three phase bridge rectifying circuits are connected in parallel, and a smoothing capacitor is connected to the output in order to suppress the ripple. In the circuit, however, a reactor is inserted in series to the three phases AC input to avoid an imbalance in the current value of each rectifying circuit, which not only makes them complicated but also includes two diodes in the channel of DC current. Therefore, the prior art circuit is not suitable for a large current application.
Among the devices connecting two rectifying circuits in parallel is a double star connection with rectifying circuits. The rectifying circuit is composed of a pair of three phase star connections with half-wave rectifying circuits connected in parallel, and has been widely used as a large current power source, including that of the consumable electrode arc welder.
FIG. 5 shows a device having the above-described construction, the operation of which is described below. A three phase AC current applied to a primary coil of a welding transformer 1 makes secondary coils 1a-1c of the star connection and secondary coils 1d-1f of the star connection generate three phases AC voltage. The AC voltages at the secondary coils 1a-1c are rectified in three phase half-wave by control rectifiers (e.g. thyristors) 2a-2c, respectively, to generate a DC voltage between a neutral point 8 and anodes of the control rectifiers (e.g. thyristors) 2a-2c. In the same way, the AC voltages at the secondary coils 1d-1f are rectified in three phase half-wave by the control rectifiers 2d-2f, respectively, to generate a DC voltage between the neutral point 8 of the star connection and anodes of control rectifiers 2d-2f. These DC voltage outputs are applied to an interphase reactor 3, to be led from the middle point 11 of the interphase reactor 3 to the negative side of output terminals 5 through DC reactor 4. The output terminals 5 connect with output cables which are connected with a welding torch or a parent metal.
Moreover, the phase difference between the AC voltage of secondary coils 1a-1c and the AC voltage of the secondary coils 1d-1f are normally set to be .pi./3 to obtain a DC current having smaller ripple equivalent to six phases. As the circuit has only one control rectifier in each of the electricity channels the voltage drop is minimum, and because of the parallel structure of two rectifying circuits this type of structure is widely used as the power source for consumable electrode arc welding which needs a large electric current.
The problem of the prior art arc welding power source is that the inductance of the DC reactor must be large in order to make the ripple in welding current small for a stable welding operation. However, when the inductance value is large a dI/dt value and a short circuit current value during welding, both of which are essential to obtain a good weld, are suppressed by the DC reactor. Furthermore, the waveform of the welding current is directly influenced by the rectified waveform of each of the control rectifiers, as shown in FIG. 6, making it impossible to obtain a stable waveform of a welding current. Furthermore, the short circuit current value that plays an important role in welding is not stable; consequently, the results of welding are not satisfactory.